This invention relates to photoflash chargers. More specifically, this invention relates to reducing input current in photoflash chargers.
A reduced average input mode—i.e., a mode in which the current flowing from a battery, for example, into a charging circuit—e.g., a switching regulator—to then be transferred to a load via the charging circuit is at a level that is less than the typical current flowing into the charging circuit—may be useful during periods when the charger does not require a relatively fast charging. One example of a situation which may use a reduced average input mode is at the end of a batteries charge, a user may want to “squeeze” a few more cycles out of the battery before the battery loses charge completely. A reduced average input mode may help accomplish this by squeezing out an additional few charging cycles, albeit at a slower pace than in a normal input current mode. The conventional method of reducing the average input current of a photoflash charger is to simply lower the current limit of the charging device.
A normal mode and a reduced mode are shown in FIG. 1. Saw tooth diagram 110 shows the behavior of the normal charging current in the primary winding of an inductor in a photoflash charger. Saw tooth diagram 120 shows a reduced average input current.
This technique of reducing the current limit of the charging device reduces the average input current. However, in order to maintain a constant level of output load, the operating frequency of the charging device is increased. The OFF-time (TOFF in FIG. 1) of the power switch of the charging device is, consequently, also smaller. This shorter OFF-time can raise certain issues.
For example, some photoflash chargers, such as the LTC 3468 and the LTC 3484 produced by Linear Technology Corporation of Milpitas, Calif., use a flyback waveform across the primary winding of a transformer in a switching regulator to do indirect detection of VOUT. When the current limit is reduced, the pulse width of the flyback waveform is also reduced. In such devices, the VOUT detection device, such as a comparator, has less time to detect when VOUT reaches a target output voltage. This may require that the primary inductance of the transformer be increased in order to maintain a predetermined minimum TOFF when the circuit is in reduced input current mode. Increasing the inductance of the primary winding is an undesirable outcome at least because it typically requires additional space on the printed circuit board to be set aside for the larger inductor.
FIG. 2 graphically illustrates the reduced OFF time that occurs during reduced input current mode. Normal mode of saw tooth diagram 210 is shown in area 212. Reduced input current mode is shown in area 214. The diagram of TOFF 216 clearly indicates the reduced TOFF time during reduced input current mode 214.
It would be desirable to provide a photoflash charger that includes a reduced average input current mode that preserves a sufficiently large TOFF period. The increased TOFF period enables measurement of the primary winding without increasing the primary winding inductance.